The present invention relates to piezoelectric crystalline films. More particularly, it relates to piezoelectric crystalline films comprising zinc oxide having a hexagonal crystal structure, and a method for making the same.
There are many methods for making piezoelectric crystalline zinc oxide films such as, for example, vacuum deposition methods, epitaxial growth methods, sputtering methods, and ion plating methods. Among these methods, the sputtering methods, particularly, radio-frequency sputtering method has been used very often recently because it has the advantage that the high growth rate of the oriented crystalline films is possible, thus making it possible to mass-produce piezoelectric crystalline films, industrially.
When making a piezoelectric crystalline film of zinc oxide on a substrate surface by the radio-frequency sputtering method, a ceramic of highly pure zinc oxide has been conventionally used as a source of the film material. However, even when radio-frequency sputtering is effected with such a film material source, it results in the formation of a crystalline film which is poor in adhesion to a substrate and low in quality. In addition, it is difficult with such a film material source to make a piezoelectric crystalline film with a c-axis perpendicular to the substrate surface. If a piezoelectric crystalline film of zinc oxide has poor adhesion, various disadvantages occur. For example, when manufacturing an acoustic surface wave filter with such a zinc oxide film, it is difficult to form interdigital transducers on the film surface, and the produced acoustic surface wave filter tends to have the disconnection of interdigital transducers, and to possess a large propagation loss of acoustic surface waves. Also, if the c-axis of the zinc oxide film is inclined with respect to the axis perpendicular to the substrate surface, the electromechanical coupling factor is lowered, thus making it difficult to produce a piezoelectric crystalline film transducer with good conversion efficiency.
It has now been found that the incorporation of uranium into a zinc oxide film makes it possible to produce a piezoelectric crystalline film with a c-axis perpendicular to the substrate surface and with high quality.
It is an object of the present invention to provide an improved zinc oxide piezoelectric crystalline film which overcomes the aforesaid disadvantages.
Another object of the present invention is to provide a method for making such improved piezoelectric crystalline films.
According to the present invention, there is provided a piezoelectric crystalline film on a substrate, the film being a crystalline zinc oxide film with a hexagonal crystal structure and a c-axis substantially perpendicular to the substrate surface, characterized in that said zinc oxide film contains uranium.
Since the content of uranium has great influence on the electrical and physical properties of the films, it is preferred to limit its content to the range of from 0.01 to 20.0 atomic percent when converted into the percentage of uranium atoms. Because, if the content of uranium is less than 0.01 atomic percent, the film changes for the worse in the quality and adhesion, and if the content of uranium is more than 20.0 atomic percent, the direction of the crystallographic orientation of the film can not be well controlled, resulting in the worsening of the orientaion of the piezoelectric crystalline film.
As a material for the substrate on which the piezoelectric crystalline film is formed, there may be used those substances such as, for example, metal, glass, ceramics, single crystals, resin, rubber and the like.
The piezoelectric crystalline film of the present invention has a c-axis substantially perpendicular to the substrate surface, thus making it possible to produce piezoelectric transducers with good conversion efficiency. Further, the piezoelectric crystalline films of the present invention have high resistivity and good adhesion, and a smooth surface.
The piezoelectric crystalline film of the present invention may be made by any sputtering method such as the radio-frequency sputtering method, and the co-sputtering method. A preferred method for making piezoelectric crystalline films according to the present invention comprises sputtering zinc oxide and uranium onto a surface of a substrate to form a crystalline zinc oxide film with a hexagonal crystal structure and a c-axis substantially perpendicular to the substrate surface, and is characterized in that said sputtering is effected by using a film material source consisting essentially of a ceramic of zinc oxide containing 0.01 to 20.0 atomic percent of uranium.